Resin compositions in which an amino formaldehyde resin (hereinafter abbreviated to an amino resin) is used as a cross-linking agent (curing agent) have so far been known as a thermosetting paint composition. Paints comprising the above resin composition are diluted with an organic solvent and then used. In recent years, thermosetting paint compositions which can be diluted with water have actively been researched for the purpose of reducing organic solvents discharged in drying and baking of coated films from the viewpoints of air pollution and environmental protection.
In general, as a water-based thermosetting paint composition in which an amino resin is used as a cross-linking agent (curing agent), mention may be made of thermosetting paint compositions obtained by combining an amino resin which is alkyl-etherified by methanol with a resin (hereinafter abbreviated to a base resin) which has a water solubility or a water dispersibility and which produces a cross-linking reaction with an amino resin. However, the thermosetting paint compositions are slow in cross-linking reaction in thermosetting thereof, and coated films obtained therefrom tend to be inferior in a water resistance as compared with coated films formed from conventional paints diluted with an organic solvent.
On the other hand, mention may be made of thermosetting paint compositions obtained by combining an amino resin which is alkyl-etherified by an alcohol such as butanol (for example, a butylated melamine resin) with a base resin as well. They are inferior in a water reducibility, but a curing property thereof and a water resistance of coated films obtained by curing them are excellent, and therefore a technique in which they are applied to water-based paints is reported as well (patent documents 1 and 2). However, since a dispersant used for stably dispersing an amino resin poor in water-solubility in water has a high hydrophilicity, eventually, use of the amino resin which is alkyl-etherified by an alcohol such as butanol does not make it possible to completely improve a water resistance of the coated film.
The amino resin described above which is alkyl-etherified by methanol is designed so that a content ratio of a mononuclear body in which one melamine skeleton is contained in one molecule of the amino resin is as high as 50% or more for the purpose of exhibiting its water solubility. Accordingly, the amino resin involves a problem that a coated film prepared by curing it is inferior in mechanical physical properties to that prepared from a butylated melamine resin having a smaller content ratio of a mononuclear body.
Further, amino resins modified with high molecular hydrophilic compounds, such as modified amino resins having polyethylene glycol or polypropylene glycol skeleton have been reported as well (patent documents 3 and 4). However, involved therein as well is problems that the part of the amino resin skeleton excluding the polyethylene glycol or polypropylene glycol skeleton which is important for maintaining performances is reduced in a proportion in a molecule due to high molecular segments for providing a water base property such as polyethylene glycol and that the expected performances are less liable to be exerted.
In addition thereto, amino resins modified with 2,2-bis-(dimethylol)butanoic acid which is an oxycarboxylic acid and the like have been invented as well (patent document 5). However, in order to produce such amino resins, the reaction temperature has to be elevated up to 100° C. in introducing an acid functional group by hydroxyl group modification. Accordingly, involved therein is manufacturing problems that self cross-linking is liable to proceed when melamine which is an amino resin used particularly for water-based paints and which has three amino groups is used as a raw material and that it is difficult to obtain an amino resin having a desired molecular weight.
Patent document 1: Japanese Patent Application Laid-Open No. 1991-21673
Patent document 2: Japanese Patent Application Laid-Open No. 1991-24170
Patent document 3: Japanese Patent Application Laid-Open No. 2002-97243
Patent document 4: Japanese Patent Application Laid-Open No. 1994-145277
Patent document 5: Japanese Patent Application Laid-Open No. 1996-165408